The present invention relates to generators having modular field windings for mounting on a rotor core and particularly relates to cooling flow directing elements for disposition between circumferentially adjacent windings to optimize radial outward cooling flow for the field windings.
Conventional generators have rotors that support field windings. These rotors typically have rotor cores with circumferentially spaced axially extending slots that receive each turn of the field windings. These rotor slots usually have wedges secured in the radial outer ends of the slots to restrain the windings against centrifugal forces that arise as the rotor spins. During assembly, the windings are installed turn by turn in the slots of the rotor core. Conventional generator field winding turns are cooled by flowing a cooling medium such as air or hydrogen through axial grooves or radial holes punched in the copper turns. Since the cooling gas path is machined, optimized cooling gas paths can be achieved to provide maximum heat transfer.
Modular field winding systems have been developed including a generator rotor having a multi-pole magnetic core and a plurality of modular winding assemblies, one for each pole. The winding assemblies are fitted over the parallel-sided forging of the multi-pole magnetic core and an enclosure is slidable over the assembly. The winding assemblies also include a series of axially spaced winding spacers or baffles that hold each of the winding turns in fixed relation to one another. The openings between the axially adjacent spacers and the circumferentially adjacent turns define natural open spaces for flowing a cooling gas, e.g., air or hydrogen, radially outwardly between the spacers and turns. Because of this arrangement, it will be appreciated that the heat transfer surface area is increased. However, the cooling gas velocity and, hence, the heat transfer coefficient is more a function of the geometry of the cooling path rather than the increased surface area. More particularly, the cooling gas path flows radially from inside the coil where the space between adjacent coils is the narrowest and flows radially outwardly to exit at the outer radius where the space between adjacent coils is the widest. The flow velocity is therefore reduced in a radial outward direction, and the convective heat transfer coefficient of the copper turns is reduced. Hotter downstream cooling gas combined with the reduced heat transfer coefficient results in higher local conductor temperatures along the outer radius of the windings.
In accordance with a preferred embodiment of the present invention, flow directing elements are inserted into the openings between the spacers and windings to provide a constant or increased flow velocity when cooling gas flows through the field windings in a radial outward direction. By maintaining or increasing the flow velocity along the radial path, the peak temperature of the windings is reduced and increased flow velocity will reduce average winding temperature. The cross-sectional area may thus remain constant, resulting in a constant flow velocity, as compared to a decreasing flow velocity in the absence of the flow directing elements and, hence, a higher comparative heat transfer coefficient. By shaping the elements such that the flow area decreases in a radial outward direction, the flow velocity and, hence, the heat transfer coefficient will increase. Consequently, the highest heat transfer coefficients may be obtained near the outer radius of the turns where the temperatures are the highest. Also, different areas between the coils can be equipped with different sized elements to provide high heat transfer coefficients in hot areas by increasing the flow velocity. By using different sized elements, flow distribution can be more uniform, resulting in a more uniform field winding temperature.
Heat transfer coefficients may be increased even further by using heat transfer augmentation surfaces or flow directing element supporting members. For example, the surfaces of the flow elements may be conditioned, such as by providing a roughness, dimples, grooves or vortex generators to effectively increase heat transfer coefficients.
In a preferred embodiment according to the present invention, there is provided a cooling system for a generator comprising a generator rotor including a multi-pole magnetic core, a plurality of modular field windings about the rotor, one for each pole, each modular field winding including a plurality of coils circumferentially spaced from one another with circumferentially adjacent coils defining generally wedge-shaped openings therebetween and flow directing elements disposed in the openings and defining with the coils passages for flowing a cooling gas from radially within the coils in a generally radial outward direction to exit locations adjacent an outer periphery of the rotor to cool the coils.
In a further preferred embodiment according to the present invention, there is provided a cooling system for a generator comprising a generator rotor, a plurality of modular field windings about the rotor, each modular field winding including a plurality of coils circumferentially spaced from one another with circumferentially adjacent coils defining openings therebetween and means disposed in the openings and between the coils for directing a cooling gas flow along the coils from radially within the coils in a generally radial outward direction to exit locations adjacent an outer periphery of the rotor to cool the coils, the cooling gas directing means being configured to provide with the circumferentially adjacent coils cooling flow passages affording a constant or increasing flow velocity to the cooling gas in a radially outward direction.